Stop resolving all of js_helper unconditionally.

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 108 matching lines @@
   void assembleProgram();
   List<CompilerTask> get tasks;
 
   // TODO(ahe,karlklose): rename this?
   void dumpInferredTypes() {}
 
   ItemCompilationContext createItemCompilationContext() {
     return new ItemCompilationContext();
   }
 
-  SourceString getCheckedModeHelper(DartType type) => null;
+  // The following methods are hooks for the backend to register its
+  // helper methods.

[ahe, 2013/02/18 12:38:04]

Categories like this doesn't work, what happens if a method is added after them?

I would prefer if you could add real documentation comments to each of the
methods.

For example:

/// Called during resolution (and SSA graph building?) to record that [cls] was
instantiated from [element].
void registerInstantiatedClass(Element user, ClassElement cls, Enqueuer world)
{}

   void registerInstantiatedClass(ClassElement cls, Enqueuer enqueuer) {}
+  void registerStringInterpolation() {}

[ahe, 2013/02/18 12:38:04]

I think all of the following methods should take the element which add the
dependency. This will help us shortly when we start looking at dynamic
recompilation.

+  void registerCatchStatement() {}
+  void registerThrow() {}
+  void registerLazyField() {}
+  void registerTypeLiteral() {}
+  void registerStackTraceInCatch() {}
+  void registerIsCheck(DartType type, Enqueuer enqueuer) {}
+  void registerAsCheck(DartType type) {}
+  void registerThrowNoSuchMethod() {}
+  void registerThrowRuntimeError() {}
+  void registerAbstractClassInstantiation() {}
+  void registerFallThroughError() {}
+  void registerSuperNoSuchMethod() {}
+  void registerConstantMap() {}
+  void registerRuntimeType() {}
 }
 
 /**
  * Key class used in [TokenMap] in which the hash code for a token is based
  * on the [charOffset].
  */
 class TokenKey {
   final Token token;
   TokenKey(this.token);
   int get hashCode => token.charOffset;
@@ skipping 983 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');
   }
 }