Disable type propagation for the dart backend.

tests/compiler/dart2js/backend_dart/opt_constprop_test.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.
 
 import 'dart:async';
 import '../mock_compiler.dart';
 import 'sexpr_unstringifier.dart';
 import 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'package:compiler/src/cps_ir/cps_ir_nodes_sexpr.dart';
@@ skipping 377 matching lines @@
   (InvokeContinuation k0 v0))
 """;
 String CP10_OUT = CP10_IN;
 
 /// Normalizes whitespace by replacing all whitespace sequences by a single
 /// space and trimming leading and trailing whitespace.
 String normalizeSExpr(String input) {
   return input.replaceAll(new RegExp(r'[ \n\t]+'), ' ').trim();
 }
 
-class UnitTypeSystem implements TypeSystem<String> {
-  static const String UNIT = 'unit';
-
-  get boolType => UNIT;
-  get dynamicType => UNIT;
-  get functionType => UNIT;
-  get intType => UNIT;
-  get listType => UNIT;
-  get mapType => UNIT;
-  get stringType => UNIT;
-  get typeType => UNIT;
-
-  bool areAssignable(a, b) => true;
-  getParameterType(_) => UNIT;
-  getReturnType(_) => UNIT;
-  join(a, b) => UNIT;
-  typeOf(_) => UNIT;
-}
-
 /// Parses the given input IR, runs an optimization pass over it, and compares
 /// the stringification of the result against the expected output.
 Future testConstantPropagator(String input, String expectedOutput) {
   final compiler = new MockCompiler.internal(
       emitJavaScript: false,
       enableMinification: false);
   return compiler.init().then((_) {
     final unstringifier = new SExpressionUnstringifier();
     final stringifier   = new SExpressionStringifier();
     final optimizer     = new TypePropagator(
@@ skipping 17 matching lines @@
   asyncTest(() => testConstantPropagator(CP2_IN, CP2_OUT));
   asyncTest(() => testConstantPropagator(CP3_IN, CP3_OUT));
   asyncTest(() => testConstantPropagator(CP4_IN, CP4_OUT));
   asyncTest(() => testConstantPropagator(CP5_IN, CP5_OUT));
   asyncTest(() => testConstantPropagator(CP6_IN, CP6_OUT));
   asyncTest(() => testConstantPropagator(CP7_IN, CP7_OUT));
   asyncTest(() => testConstantPropagator(CP8_IN, CP8_OUT));
   asyncTest(() => testConstantPropagator(CP9_IN, CP9_OUT));
   asyncTest(() => testConstantPropagator(CP10_IN, CP10_OUT));
 }