Fix typechecks for constant assignments of function to typedefs.

sdk/lib/_internal/compiler/implementation/constants.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 dart2js;
 
 abstract class ConstantVisitor<R> {
   R visitFunction(FunctionConstant constant);
   R visitNull(NullConstant constant);
   R visitInt(IntConstant constant);
@@ skipping 57 matching lines @@
     if (other is !FunctionConstant) return false;
     return identical(other.element, element);
   }
 
   String toString() => element.toString();
   List<Constant> getDependencies() => const <Constant>[];
   DartString toDartString() {
     return new DartString.literal(element.name);
   }
 
-  DartType computeType(Compiler compiler) {
-    return compiler.functionClass.computeType(compiler);
-  }
+  // TODO(johnniwinther): remove computeType.
+  DartType computeType(Compiler compiler) => element.computeType(compiler);
 
   ti.TypeMask computeMask(Compiler compiler) {
     return compiler.typesTask.functionType;
   }
 
   int get hashCode => (17 * element.hashCode) & 0x7fffffff;
 
   accept(ConstantVisitor visitor) => visitor.visitFunction(this);
 }
 
@@ skipping 542 matching lines @@
       if (i > 0) sb.write(',');
       sb.write(Error.safeToString(field.name));
       sb.write('=');
       sb.write(Error.safeToString(value));
       i++;
     });
     sb.write('))');
     return sb.toString();
   }
 }