Disable type propagation for the dart backend.

pkg/compiler/lib/src/dart_backend/backend.dart

Index: pkg/compiler/lib/src/dart_backend/backend.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend.dart b/pkg/compiler/lib/src/dart_backend/backend.dart
index 6f8ba84712b2400534f94c4bf9a6a4821d305ded..f6f67484d564a58db8f0f43fdc0f789a9be92953 100644
--- a/pkg/compiler/lib/src/dart_backend/backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend.dart
@@ -146,7 +146,9 @@ class DartBackend extends Backend {
       }
     }
 
-    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);