dart2js: support tear-offs in the startup emitter.

pkg/compiler/lib/src/js_emitter/code_emitter_task.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.js_emitter;
 
 const USE_LAZY_EMITTER = const bool.fromEnvironment("dart2js.use.lazy.emitter");
 const USE_STARTUP_EMITTER =
     const bool.fromEnvironment("dart2js.use.startup.emitter");
 
@@ skipping 35 matching lines @@
       emitter = new startup_js_emitter.Emitter(compiler, namer, nativeEmitter);
     } else {
       emitter =
           new full_js_emitter.Emitter(compiler, namer, generateSourceMap, this);
     }
     metadataCollector = new MetadataCollector(compiler, emitter);
   }
 
   String get name => 'Code emitter';
 
+  /// Returns the string that is used to find library patches that are
+  /// specialized for the emitter.
+  String get patchVersion => emitter.patchVersion;
+
   /// Returns the closure expression of a static function.
   jsAst.Expression isolateStaticClosureAccess(FunctionElement element) {
     return emitter.isolateStaticClosureAccess(element);
   }
 
   /// Returns the JS function that must be invoked to get the value of the
   /// lazily initialized static.
   jsAst.Expression isolateLazyInitializerAccess(FieldElement element) {
     return emitter.isolateLazyInitializerAccess(element);
   }
@@ skipping 75 matching lines @@
           compiler, namer, this, emitter, rtiNeededClasses);
       int size = emitter.emitProgram(programBuilder);
       // TODO(floitsch): we shouldn't need the `neededClasses` anymore.
       neededClasses = programBuilder.collector.neededClasses;
       return size;
     });
   }
 }
 
 abstract class Emitter {
+  /// Returns the string that is used to find library patches that are
+  /// specialized for this emitter.
+  String get patchVersion;
+
   /// Uses the [programBuilder] to generate a model of the program, emits
   /// the program, and returns the size of the generated output.
   int emitProgram(ProgramBuilder programBuilder);
 
   /// Returns true, if the emitter supports reflection.
   bool get supportsReflection;
 
   /// Returns the JS function that must be invoked to get the value of the
   /// lazily initialized static.
   jsAst.Expression isolateLazyInitializerAccess(FieldElement element);
@@ skipping 32 matching lines @@
   bool isConstantInlinedOrAlreadyEmitted(ConstantValue constant);
 
   /// Returns the JS code for accessing the given [constant].
   jsAst.Expression constantReference(ConstantValue constant);
 
   /// Returns the JS template for the given [builtin].
   jsAst.Template templateForBuiltin(JsBuiltin builtin);
 
   void invalidateCaches();
 }