Create an option to disable type inference and type promotion.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

 // Copyright (c) 2016, 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 fasta.source_library_builder;
 
 import 'package:front_end/src/scanner/token.dart' show Token;
 
 import 'package:kernel/ast.dart' show ProcedureKind;
 
@@ skipping 40 matching lines @@
   final List<SourceLibraryBuilder<T, R>> parts = <SourceLibraryBuilder<T, R>>[];
 
   final List<Import> imports = <Import>[];
 
   final Scope importScope;
 
   final Uri fileUri;
 
   final List<List> implementationBuilders = <List<List>>[];
 
+  /// Indicates whether type inference (and type promotion) should be disabled
+  /// for this library.
+  final bool disableTypeInference;
+
   String name;
 
   String partOfName;
 
   Uri partOfUri;
 
   List<MetadataBuilder> metadata;
 
   /// The current declaration that is being built. When we start parsing a
   /// declaration (class, method, and so on), we don't have enough information
   /// to create a builder and this object records its members and types until,
   /// for example, [addClass] is called.
   DeclarationBuilder<T> currentDeclaration;
 
   bool canAddImplementationBuilders = false;
 
   SourceLibraryBuilder(SourceLoader loader, Uri fileUri)
       : this.fromScopes(loader, fileUri, new DeclarationBuilder<T>.library(),
             new Scope.top());
 
   SourceLibraryBuilder.fromScopes(
       this.loader, this.fileUri, this.libraryDeclaration, this.importScope)
-      : currentDeclaration = libraryDeclaration,
+      : disableTypeInference = loader.target.disableTypeInference,
+        currentDeclaration = libraryDeclaration,
         super(
             fileUri, libraryDeclaration.toScope(importScope), new Scope.top());
 
   Uri get uri;
 
   bool get isPart => partOfName != null || partOfUri != null;
 
   bool get isPatch;
 
   List<T> get types => libraryDeclaration.types;
@@ skipping 505 matching lines @@
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 
   Scope toScope(Scope parent) {
     return new Scope(members, setters, parent, isModifiable: false);
   }
 }