Disable type propagation for the dart backend.

tests/compiler/dart2js/backend_dart/opt_constprop_test.dart

Index: tests/compiler/dart2js/backend_dart/opt_constprop_test.dart
diff --git a/tests/compiler/dart2js/backend_dart/opt_constprop_test.dart b/tests/compiler/dart2js/backend_dart/opt_constprop_test.dart
index bd989ef589f079f9f78c132d84be45e01f2eb919..fc35ed0c2bd5de5882295806d93df53d41e939f9 100644
--- a/tests/compiler/dart2js/backend_dart/opt_constprop_test.dart
+++ b/tests/compiler/dart2js/backend_dart/opt_constprop_test.dart
@@ -395,25 +395,6 @@ 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) {