Disable source maps completely.

dart/pkg/compiler/lib/src/js_backend/codegen/task.dart

 // Copyright (c) 2014, 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.
 
 /// Generate code using the cps-based IR pipeline.
 library code_generator_task;
 
 import '../js_backend.dart';import '../../dart2jslib.dart';
 import '../../source_file.dart';
 import '../../cps_ir/cps_ir_nodes.dart' as cps;
@@ skipping 27 matching lines @@
 
   final ConstantSystem constantSystem;
 
   final Compiler compiler;
 
   // Remember to update dart-doc of [compile] when this field is removed.
   FunctionCompiler fallbackCompiler;
 
   CspFunctionCompiler(Compiler compiler, JavaScriptBackend backend)
       : irBuilderTask = new IrBuilderTask(compiler),
-        fallbackCompiler = new ssa.SsaFunctionCompiler(backend),
+        fallbackCompiler = new ssa.SsaFunctionCompiler(backend, true),
         constantSystem = backend.constantSystem,
         compiler = compiler;
 
   /// Generates JavaScript code for `work.element`. First tries to use the
   /// Cps Ir -> tree ir -> js pipeline, and if that fails due to language
   /// features not implemented it will fall back to the ssa pipeline.
   js.Fun compile(CodegenWorkItem work) {
     AstElement element = work.element;
       return compiler.withCurrentElement(element, () {
         try {
@@ skipping 102 matching lines @@
   SourceFile sourceFileOfElement(Element element) {
     return element.implementation.compilationUnit.script.file;
   }
 
   js.Fun buildJavaScriptFunction(FunctionElement element,
                                  List<js.Parameter> parameters,
                                  js.Block body) {
     return attachPosition(new js.Fun(parameters, body), element);
   }
 }