kernel -> ssa: implement if-statements

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 1c4c68973faa4310e27fda94835ffcc5c83a6cf5..9d87ad808aaf7284e7bb8156f74dcbdc25a92e81 100644
--- a/pkg/compiler/lib/src/ssa/graph_builder.dart
+++ b/pkg/compiler/lib/src/ssa/graph_builder.dart
@@ -4,6 +4,8 @@
 
 import '../elements/elements.dart';
 import '../types/types.dart';
+
+import 'locals_handler.dart';
 import 'nodes.dart';
 
 /// Base class for objects that build up an SSA graph.
@@ -14,6 +16,9 @@ abstract class GraphBuilder {
   /// Holds the resulting SSA graph.
   final HGraph graph = new HGraph();
 
+  /// Used to track the locals while building the graph.
+  LocalsHandler localsHandler;
+
   /// A stack of instructions.
   ///
   /// We build the SSA graph by simulating a stack machine.
@@ -29,6 +34,14 @@ abstract class GraphBuilder {
     return stack.removeLast();
   }
 
+  /// Pops the most recent instruction from the stack and 'boolifies' it.
+  ///
+  /// Boolification is checking if the value is '=== true'.
+  HBoolify popBoolified();
+
+  /// Pushes a boolean checking [expression] against null.
+  pushCheckNull(HInstruction expression);
+
   void dup() {
     stack.add(stack.last);
   }