dart2js cps_ir: Seed type_propagation with constant bools from inference.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index 8c37b3ac201a5800178f8ce0a506e976b0d70912..383b39fb68ce20471bb7b6a76aadfcea68172dd6 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -321,7 +321,13 @@ class ConstantPropagationLattice {
   /// [typedSelector]. If the given selector is not a [TypedSelector], any
   /// reachable method matching the selector may be targeted.
   AbstractValue getInvokeReturnType(Selector selector, TypeMask mask) {
-    return nonConstant(typeSystem.getInvokeReturnType(selector, mask));
+    return fromMask(typeSystem.getInvokeReturnType(selector, mask));
+  }
+
+  AbstractValue fromMask(TypeMask mask) {
+    ConstantValue constantValue = typeSystem.getConstantOf(mask);
+    if (constantValue != null) return constant(constantValue, mask);
+    return nonConstant(mask);
   }
 }
 
@@ -2066,7 +2072,7 @@ class TypePropagationVisitor implements Visitor {
       TypeMask type = param.hint is ParameterElement
           ? typeSystem.getParameterType(param.hint)
           : typeSystem.dynamicType;
-      setValue(param, nonConstant(type));
+      setValue(param, lattice.fromMask(type));
     }
     push(node.body);
   }
@@ -2113,7 +2119,7 @@ class TypePropagationVisitor implements Visitor {
     }
 
     TypeMask returnType = typeSystem.getReturnType(node.target);
-    setResult(node, nonConstant(returnType));
+    setResult(node, lattice.fromMask(returnType));
   }
 
   void visitInvokeContinuation(InvokeContinuation node) {