kernel -> ssa: implement if-statements

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

 // Copyright (c) 2016, 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.
 
 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.
 ///
 /// This contains helpers for building the graph and tracking information about
 /// the current state of the graph being built.
 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.
   List<HInstruction> stack = <HInstruction>[];
 
   void push(HInstruction instruction) {
     add(instruction);
     stack.add(instruction);
   }
 
   HInstruction pop() {
     return stack.removeLast();
   }
 
+  /// Pops the most recent instruction from the stack and 'boolifies' it.
+  ///
+  /// Boolification is checking if the value is '=== true'.
+  HInstruction popBoolified();
+
+  /// Pushes a boolean checking [expression] against null.
+  pushCheckNull(HInstruction expression);
+
   void dup() {
     stack.add(stack.last);
   }
 
   HBasicBlock _current;
 
   /// The current block to add instructions to. Might be null, if we are
   /// visiting dead code, but see [isReachable].
   HBasicBlock get current => _current;
 
@@ skipping 73 matching lines @@
     HParameterValue result = new HParameterValue(parameter, type);
     if (lastAddedParameter == null) {
       graph.entry.addBefore(graph.entry.first, result);
     } else {
       graph.entry.addAfter(lastAddedParameter, result);
     }
     lastAddedParameter = result;
     return result;
   }
 }