implement kernel -> ssa for function that returns a constant

pkg/compiler/lib/src/ssa/graph_builder.dart

Index: pkg/compiler/lib/src/ssa/graph_builder.dart
diff --git a/pkg/compiler/lib/src/ssa/graph_builder.dart b/pkg/compiler/lib/src/ssa/graph_builder.dart
index 969899f617e6e1032d0ca4031d2e56906a95b95d..1c4c68973faa4310e27fda94835ffcc5c83a6cf5 100644
--- a/pkg/compiler/lib/src/ssa/graph_builder.dart
+++ b/pkg/compiler/lib/src/ssa/graph_builder.dart
@@ -14,6 +14,25 @@ abstract class GraphBuilder {
   /// Holds the resulting SSA graph.
   final HGraph graph = new HGraph();
 
+  /// A stack of instructions.
+  ///
+  /// We build the SSA graph by simulating a stack machine.
+  List<HInstruction> stack = <HInstruction>[];
+
+
+  void push(HInstruction instruction) {
+    add(instruction);
+    stack.add(instruction);
+  }
+
+  HInstruction pop() {
+    return stack.removeLast();
+  }
+
+  void dup() {
+    stack.add(stack.last);
+  }
+
   HBasicBlock _current;
 
   /// The current block to add instructions to. Might be null, if we are