Related to #1284

The test ensures that the property is there, because it is important to
some people.

Fixes #1284

Appearently, there is a use-case of stringifying the error in order to
evaluated its properties on another system. There was a regression
from 4.0.5  to 4.0.6 that the column-property of compilation errors
was not  enumerable anymore in 4.0.6 (due to  commit 20c965c) and
thus was not included in the output of "JSON.stringify".

(node:30288) DeprecationWarning: sys is deprecated. Use util instead.

(cherry picked from commit 9a36966 by @travnels)

Avoid duplicate // sourceMappingURL=... lines when minifying AND
generating a map. UglifyJS2 will write the line when minifying.

(cherry picked from commit 660a117)

Fixes #1252
- This fix treats partial-blocks more like closures and uses the closure-context of
  the "invokePartial"-function to store the @partial-block for the partial.
- Adds a tes for the fix

- Multiple partial-blocks at different nesting levels
- Calling partial-blocks twice with nested partial-blocks
- Calling the partial-block from within the #each-helper
- nested inline partials with partial-blocks on different nesting levels
- nested inline partials (twice at each level)

- This export will be transpiled by Babel for the cjs distribution,
  but will enable others to  use a pure ES6 module distribution
- Instanbul: Ignore "parser.js" for coverage reporting. This file was ignored before
  via annotation, but this has no effect anymore due to the above change
- Remove istanbul annotation from `parser-prefix` (@nknapp)

Squashed by @nknapp

(cherry picked from commit 508347e)

Fixes #1319

Original behaviour:
- When a block-helper was called on a null-context, an empty object was used
  as context instead. (#1093)
- The runtime verifies that whether the current context equals the
  last context and adds the current context to the stack, if it is not.
  This is done, so that inside a block-helper, the ".." path can be used
  to go back to the parent element.
- If the helper is called on a "null" element, the context was added, even
  though it shouldn't be, because the "null != {}"

Fix:
- The commit replaces "null" by the identifiable "container.nullContext"
  instead of "{}". "nullContext" is a sealed empty object.
- An additional check in the runtime verifies that the context is
  only added to the stack, if it is not the nullContext.

Backwards compatibility within 4.0.x-versions:
- This commit changes the compiler and compiled templates would not work
  with runtime-versions 4.0.0 - 4.0.6, because of the "nullContext"
  property. That's way, the compiled code reads
  "(container.nullContext || {})" so that the behavior will degrade
  gracefully with older runtime versions: Everything else will work
  fine, but GH-1319 will still be broken, if you use a newer compiler
  with a pre 4.0.7 runtime.