Disable type propagation for the dart backend.

pkg/compiler/lib/src/dart_backend/backend.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 dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 128 matching lines @@
     if (tracer != null) {
       tracer.traceCompilation(element.name, null);
     }
 
     void traceGraph(String title, var irObject) {
       if (tracer != null) {
         tracer.traceGraph(title, irObject);
       }
     }
 
-    new TypePropagator(compiler, constantSystem, new TypeMaskSystem(compiler),
+    // TODO(karlklose): enable type propagation for dart2dart when constant
+    // types are correctly marked as instantiated (Issue 21880).
+    new TypePropagator(compiler, constantSystem, new UnitTypeSystem(),
         compiler.internalError).rewrite(cpsDefinition);
     traceGraph("Sparse constant propagation", cpsDefinition);
     new RedundantPhiEliminator().rewrite(cpsDefinition);
     traceGraph("Redundant phi elimination", cpsDefinition);
     new ShrinkingReducer().rewrite(cpsDefinition);
     traceGraph("Shrinking reductions", cpsDefinition);
 
     // Do not rewrite the IR after variable allocation.  Allocation
     // makes decisions based on an approximation of IR variable live
     // ranges that can be invalidated by transforming the IR.
@@ skipping 357 matching lines @@
   }
 
   ConstantExpression compileMetadata(MetadataAnnotation metadata,
                            Node node,
                            TreeElements elements) {
     return measure(() {
       return constantCompiler.compileMetadata(metadata, node, elements);
     });
   }
 }