Add a new flag to dart2js: --trust-type-annotations and implement it in the types inferrer.

sdk/lib/_internal/compiler/implementation/compiler.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 233 matching lines @@
    * correspond to a particular element.
    *
    * We should get rid of this and ensure that all dependencies are
    * associated with a particular element.
    */
   final TreeElements globalDependencies = new TreeElementMapping(null);
 
   final bool enableMinification;
   final bool enableTypeAssertions;
   final bool enableUserAssertions;
+  final bool trustTypeAnnotations;
   final bool enableConcreteTypeInference;
   /**
    * The maximum size of a concrete type before it widens to dynamic during
    * concrete type inference.
    */
   final int maxConcreteTypeSize;
   final bool analyzeAll;
   final bool analyzeOnly;
   /**
    * If true, skip analysis of method bodies and field initializers. Implies
@@ skipping 167 matching lines @@
   static const int PHASE_RESOLVING = 1;
   static const int PHASE_DONE_RESOLVING = 2;
   static const int PHASE_COMPILING = 3;
   int phase;
 
   bool compilationFailed = false;
 
   bool hasCrashed = false;
 
   Compiler({this.tracer: const Tracer(),
-            this.enableTypeAssertions: false,
+            bool enableTypeAssertions: false,
             this.enableUserAssertions: false,
+            bool trustTypeAnnotations: false,
             this.enableConcreteTypeInference: false,
             this.maxConcreteTypeSize: 5,
             this.enableMinification: false,
             this.enableNativeLiveTypeAnalysis: false,
             bool emitJavaScript: true,
             bool generateSourceMap: true,
             bool disallowUnsafeEval: false,
             this.analyzeAll: false,
             bool analyzeOnly: false,
             bool analyzeSignaturesOnly: false,
             this.rejectDeprecatedFeatures: false,
             this.checkDeprecationInSdk: false,
             this.preserveComments: false,
             this.verbose: false,
             this.sourceMapUri: null,
             this.buildId: "build number could not be determined",
             outputProvider,
             List<String> strips: const []})
       : this.analyzeOnly = analyzeOnly || analyzeSignaturesOnly,
         this.analyzeSignaturesOnly = analyzeSignaturesOnly,
+        this.enableTypeAssertions = enableTypeAssertions,
+        this.trustTypeAnnotations =

[kasperl, 2013/05/16 13:17:20]

Maybe keep the flags separate here, so the inferrer can use them separately if
it needs to?

[ngeoffray, 2013/05/17 14:34:25]

Done.

+            enableTypeAssertions || trustTypeAnnotations,
         this.outputProvider =
             (outputProvider == null) ? NullSink.outputProvider : outputProvider
 
   {
     world = new World(this);
 
     closureMapping.ClosureNamer closureNamer;
     if (emitJavaScript) {
       js_backend.JavaScriptBackend jsBackend =
           new js_backend.JavaScriptBackend(this, generateSourceMap,
@@ skipping 858 matching lines @@
 
   void close() {}
 
   toString() => name;
 
   /// Convenience method for getting an [api.CompilerOutputProvider].
   static NullSink outputProvider(String name, String extension) {
     return new NullSink('$name.$extension');
   }
 }