Adds typechecking of return values

frog/gen.dart

 // Copyright (c) 2011, 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.
 
 /**
  * Top level generator object for writing code and keeping track of
  * dependencies.
  *
  * Should have two compilation models, but only one implemented so far.
  *
@@ skipping 1082 matching lines @@
     meth.generator.writeDefinition(writer, null);
     return false;
   }
 
   /**
    * Returns true indicating that normal control-flow is interrupted by
    * this statement. (This could be a return, break, throw, or continue.)
    */
   bool visitReturnStatement(ReturnStatement node) {
     if (node.value == null) {
+      // This is essentially "return null".
+      // It can't issue a warning because every type is nullable.
       writer.writeln('return;');
     } else {
       if (method.isConstructor) {
         world.error('return of value not allowed from constructor', node.span);
       }
-      writer.writeln('return ${visitValue(node.value).code};');
+      var value = visitTypedValue(node.value, method.returnType);
+      writer.writeln('return ${value.code};');
     }
     return true;
   }
 
   bool visitThrowStatement(ThrowStatement node) {
     // Dart allows throwing anything, just like JS
     if (node.value != null) {
       var value = visitValue(node.value);
       // Ensure that we generate a toString() method for things that we throw
       value.invoke(this, 'toString', node, Arguments.EMPTY);
@@ skipping 1108 matching lines @@
     }
     for (int i = bareCount; i < length; i++) {
       var name = getName(i);
       if (name == null) name = '\$$i';
       // TODO(jimhug): Need source locations.
       result.add(new Value(world.varType, name, null, false, /*needsTemp:*/false));
     }
     return new Arguments(nodes, result);
   }
 }