Implement super calls.

frog/leg/tree/nodes.dart

Index: frog/leg/tree/nodes.dart
diff --git a/frog/leg/tree/nodes.dart b/frog/leg/tree/nodes.dart
index f0c8c111c4308b5667c2d6e9bc4295be62dfbccc..f8596a3d9492b6da9e555c3030507907bf1ad527 100644
--- a/frog/leg/tree/nodes.dart
+++ b/frog/leg/tree/nodes.dart
@@ -220,6 +220,11 @@ class Send extends Expression {
 
   int argumentCount() => argumentsNode.length();
 
+  bool get isSuperCall() {
+    return receiver !== null &&
+           receiver.asIdentifier() !== null &&
+           receiver.asIdentifier().isSuper();
+  }
   bool get isOperator() => selector is Operator;
   bool get isPropertyAccess() => argumentsNode === null;
   bool get isFunctionObjectInvocation() => selector === null;
@@ -656,6 +661,8 @@ class Identifier extends Expression {
 
   bool isThis() => source.stringValue == 'this';
 
+  bool isSuper() => source.stringValue == 'super';
+
   Identifier asIdentifier() => this;
 
   accept(Visitor visitor) => visitor.visitIdentifier(this);