@@ -125,8 +125,7 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType> {
         baseElement.isSynthetic &&
         !baseElement.variable.isSynthetic) {
       var variable = baseElement.variable;
-      var decoratedElementType =
-          _variables.decoratedElementType(variable, create: true);
+      var decoratedElementType = _variables.decoratedElementType(variable);
       if (baseElement.isGetter) {
         decoratedBaseType = DecoratedType(baseElement.type, _graph.never,
             returnType: decoratedElementType);
@@ -136,8 +135,7 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType> {
             positionalParameters: [decoratedElementType]);
       }
     } else {
-      decoratedBaseType =
-          _variables.decoratedElementType(baseElement, create: true);
+      decoratedBaseType = _variables.decoratedElementType(baseElement);
     }
     if (substitution != null) {
       DartType elementType;
@@ -302,8 +300,8 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType> {
           .substitute(_decoratedClassHierarchy
               .getDecoratedSupertype(classElement, superElement)
               .asSubstitution);
-      var superConstructorDecoratedType = _variables
-          .decoratedElementType(superConstructorElement, create: true);
+      var superConstructorDecoratedType =
+          _variables.decoratedElementType(superConstructorElement);
       var origin = ImplicitMixinSuperCallOrigin(_source, node.offset);
       _unionDecoratedTypeParameters(
           constructorDecoratedType, superConstructorDecoratedType, origin);
@@ -828,15 +826,13 @@ $stackTrace''');
         for (int i = 0; i < instantiatedType.typeArguments.length; i++) {
           _unionDecoratedTypes(
               instantiatedType.typeArguments[i],
-              _variables.decoratedElementType(element.typeParameters[i],
-                  create: true),
+              _variables.decoratedElementType(element.typeParameters[i]),
               origin);
         }
       } else {
         for (int i = 0; i < typeArguments.length; i++) {
           DecoratedType bound;
-          bound = _variables.decoratedElementType(element.typeParameters[i],
-              create: true);
+          bound = _variables.decoratedElementType(element.typeParameters[i]);
           var argumentType =
               _variables.decoratedTypeAnnotation(_source, typeArguments[i]);
           if (argumentType == null) {
 
@@ -499,9 +499,10 @@ abstract class VariableRepository {
   /// Retrieves the [DecoratedType] associated with the static type of the given
   /// [element].
   ///
-  /// If [create] is `true`, and no decorated type is found for the given
-  /// element, one is synthesized using [DecoratedType.forElement].
-  DecoratedType decoratedElementType(Element element, {bool create: false});
+  /// If no decorated type is found for the given element, and the element is in
+  /// a library that's not being migrated, a decorated type is synthesized using
+  /// [DecoratedType.forElement].
+  DecoratedType decoratedElementType(Element element);
 
   /// Gets the [DecoratedType] associated with the given [typeAnnotation].
   DecoratedType decoratedTypeAnnotation(
 
@@ -37,10 +37,8 @@ class Variables implements VariableRecorder, VariableRepository {
   }
 
   @override
-  DecoratedType decoratedElementType(Element element, {bool create: false}) =>
-      _decoratedElementTypes[element] ??= create
-          ? DecoratedType.forElement(element, _graph)
-          : throw StateError('No element found');
+  DecoratedType decoratedElementType(Element element) =>
+      _decoratedElementTypes[element] ??= _createDecoratedElementType(element);
 
   @override
   DecoratedType decoratedTypeAnnotation(
@@ -77,6 +75,7 @@ class Variables implements VariableRecorder, VariableRepository {
   }
 
   void recordDecoratedElementType(Element element, DecoratedType type) {
+    assert(_graph.isBeingMigrated(element.library.source));
     _decoratedElementTypes[element] = type;
   }
 
@@ -155,6 +154,14 @@ class Variables implements VariableRecorder, VariableRepository {
     (_potentialModifications[source] ??= []).add(potentialModification);
   }
 
+  DecoratedType _createDecoratedElementType(Element element) {
+    if (_graph.isBeingMigrated(element.library.source)) {
+      throw StateError('A decorated type for $element should have been stored '
+          'by the NodeBuilder via recordDecoratedElementType');
+    }
+    return DecoratedType.forElement(element, _graph);
+  }
+
   /// Creates an entry [_decoratedDirectSupertypes] for an already-migrated
   /// class.
   Map<ClassElement, DecoratedType> _decorateDirectSupertypes(
 
@@ -21,6 +21,7 @@ class AbstractSingleUnitTest extends AbstractContextTest {
 
   String testCode;
   String testFile;
+  Uri testUri;
   Source testSource;
   ResolvedUnitResult testAnalysisResult;
   CompilationUnit testUnit;
@@ -35,7 +36,7 @@ class AbstractSingleUnitTest extends AbstractContextTest {
   }
 
   Future<void> resolveTestUnit(String code) async {
-    addTestSource(code);
+    addTestSource(code, testUri);
     testAnalysisResult = await session.getResolvedUnit(testFile);
     testUnit = testAnalysisResult.unit;
     if (verifyNoTestUnitErrors) {
@@ -59,5 +60,6 @@ class AbstractSingleUnitTest extends AbstractContextTest {
   void setUp() {
     super.setUp();
     testFile = convertPath('/home/test/lib/test.dart');
+    testUri = Uri.parse('package:test/test.dart');
   }
 }