[tool][web] Intercept dart2wasm exit code 254 cleanly and provide JS interop migration guidance for Wasm & dry-runs #190466

Currently, when compiling a web project with flutter build web --wasm (or evaluating compat via dry-run) that encounters incompatible legacy JS interop dependencies (such as dart:html or package:js), the terminal output suffers from two usability defects:

1. Uncaught ProcessException Dump (Standard Wasm Builds): flutter_tools invokes ProcessUtils.run(throwOnError: !compilerConfig.dryRun). Expected compiler failures (exit code 254) during standard --wasm builds are treated as unexpected tool exceptions, dumping a 2,000+ character compiler sub-command string and a 22-frame internal Dart tool stack trace (_DefaultProcessUtils.run, Dart2WasmTarget.build, etc.). This buries the useful dependency tree context generated by the Dart compiler under noise and makes user error look like a tool crash.
2. Missing Actionable Remediation (Wasm Builds & Dry-Runs): When compilation aborts or dry-runs detect legacy web libraries (dart:html, package:js), zero explicit guidance is given on how to migrate to WebAssembly-compatible packages (dry-runs only print a generic docs URL).

Proposed Changes

1. Modify Dart2WasmTarget.build to invoke ProcessUtils.run(throwOnError: false).
2. On non-zero exit codes during non-dry-run builds, directly forward uncorrupted compiler stdout and stderr to environment.logger without throwing an unformatted ProcessException.
3. Add a new shortlink helper constant in lib/src/web/web_constants.dart alongside kWasmMoreInfo targeting https://flutter.dev/to/wasm-errors (which will redirect to https://docs.flutter.dev/platform-integration/web/wasm#diagnosing-wasm-compilation-errors).
4. In both fatal --wasm compilations and Wasm dry-run finding outputs (_handleDryRunResult), inspect output streams for instances of 'dart:html' or 'package:js'. When present, append an educational remediation footer:

   Note: WebAssembly compilation failed due to legacy web imports. 
   Migrate your project from dart:html and package:js to package:web and dart:js_interop.
   See https://flutter.dev/to/wasm-errors for diagnostic and migration guidance.

5. For fatal --wasm builds, terminate execution cleanly via throwToolExit('Failed to compile application for the Web.'), preventing internal tool stack trace spillage.