Implement for-in.

frog/leg/resolver.dart

Index: frog/leg/resolver.dart
diff --git a/frog/leg/resolver.dart b/frog/leg/resolver.dart
index 0719b23532b118b0a745d247f5e136ca5e7ec904..144c080424de59d5ce78169aa28a0e9a8fb2c6e8 100644
--- a/frog/leg/resolver.dart
+++ b/frog/leg/resolver.dart
@@ -489,7 +489,19 @@ class FullResolverVisitor extends ResolverVisitor {
   }
 
   visitForInStatement(ForInStatement node) {
-    cancel(node, 'unimplemented');
+    visit(node.expression);
+    Scope scope = new BlockScope(context);
+    Node declaration = node.declaredIdentifier;
+    visitIn(declaration, scope);
+    visitIn(node.body, scope);
+    if (declaration is !VariableDefinitions ||
+        !declaration.definitions.nodes.tail.isEmpty()) {
+      // The variable declaration is either not a declaration, or it's
+      // declaring more than one variable.
+      // TODO(lrn): A non-terminating error report would be fine here.
+      print(declaration);
+      cancel(node.declaredIdentifier, "Invalid variable declaration");
+    }
   }
 
   visitLabelledStatement(LabelledStatement node) {