dart2cps: Fix bug in constructors.

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index aebd19dbd5fa85b098d1f1d28049b93dde0091cc..5272a196baf13e5c6942318be34a4abc28459a65 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -2581,9 +2581,8 @@ class JsIrBuilderVisitor extends IrBuilderVisitor {
     // cannot add a BoxLocal as parameter, because BoxLocal is not an element.
     // Instead of forging ParameterElements to forge a FunctionSignature, we
     // need a way to create backend methods without creating more fake elements.
-
+    constructor = constructor.implementation;

[floitsch, 2015/06/22 11:46:53]

I think we should always have implementations in the cps_ir_builder task.
From what I can see the constructor comes from a constructor-list, which in turn
is filled in evaluateConstructorFieldInitializers.
We should just get the implementation there.
(In my checkout this would be line 2437).

     assert(constructor.isGenerativeConstructor);
-    assert(invariant(constructor, constructor.isImplementation));
     if (constructor.isSynthesized) return null;
     ast.FunctionExpression node = constructor.node;
     // If we know the body doesn't have any code, we don't generate it.