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

sdk/lib/_internal/compiler/implementation/apiimpl.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.
 
 library leg_apiimpl;
 
 import 'dart:uri';
 import 'dart:async';
 
 import '../compiler.dart' as api;
@@ skipping 20 matching lines @@
            this.packageRoot,
            List<String> options)
       : this.options = options,
         this.allowedLibraryCategories = getAllowedLibraryCategories(options),
         super(
             tracer: new ssa.HTracer(
                 ssa.GENERATE_SSA_TRACE ? outputProvider('dart', 'cfg') : null),
             outputProvider: outputProvider,
             enableTypeAssertions: hasOption(options, '--enable-checked-mode'),
             enableUserAssertions: hasOption(options, '--enable-checked-mode'),
+            trustTypeAnnotations:
+                hasOption(options, '--trust-type-annotations'),
             enableMinification: hasOption(options, '--minify'),
             enableNativeLiveTypeAnalysis:
                 !hasOption(options, '--disable-native-live-type-analysis'),
             emitJavaScript: !hasOption(options, '--output-type=dart'),
             disallowUnsafeEval: hasOption(options, '--disallow-unsafe-eval'),
             analyzeAll: hasOption(options, '--analyze-all'),
             analyzeOnly: hasOption(options, '--analyze-only'),
             analyzeSignaturesOnly:
                 hasOption(options, '--analyze-signatures-only'),
             rejectDeprecatedFeatures:
@@ skipping 229 matching lines @@
       handler(translateUri(span.uri, null), span.begin, span.end,
               message, kind);
     }
   }
 
   bool get isMockCompilation {
     return mockableLibraryUsed
       && (options.indexOf('--allow-mock-compilation') != -1);
   }
 }