dart2js cps: Emit 'return' after throwing call to assist VM analysis.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index 487eb4508fd26b79eba8879da264ec83b0baac8f..8bb26fd773bd3439b8db3e8c9a99466e54209db6 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -68,6 +68,18 @@ class CodeGenerator extends tree_ir.StatementVisitor
   final tree_ir.FallthroughStack shortContinue =
       new tree_ir.FallthroughStack();
 
+  /// When the top element is true, [Unreachable] statements will be emitted
+  /// as [Return]s, otherwise they are emitted as empty because they are
+  /// followed by the end of the method.
+  ///
+  /// Note on why the [fallthrough] stack should not be used for this:
+  /// Ordinary statements may choose whether to use the [fallthrough] target,
+  /// and the choice to do so may disable an optimization in [visitIf].
+  /// But omitting an unreachable 'return' should have lower priority than
+  /// the optimizations in [visitIf], so [visitIf] will instead tell the
+  /// [Unreachable] statements whether they may use fallthrough or not.
+  List<bool> emitUnreachableAsReturn = <bool>[false];
+
   Set<tree_ir.Label> usedLabels = new Set<tree_ir.Label>();
 
   List<js.Statement> accumulator = new List<js.Statement>();
@@ -534,16 +546,34 @@ class CodeGenerator extends tree_ir.StatementVisitor
 
   @override
   void visitExpressionStatement(tree_ir.ExpressionStatement node) {
-    accumulator.add(new js.ExpressionStatement(
-        visitExpression(node.expression)));
-    visitStatement(node.next);
+    js.Expression exp = visitExpression(node.expression);
+    if (node.next is tree_ir.Unreachable && emitUnreachableAsReturn.last) {
+      // Emit as 'return exp' to assist local analysis in the VM.
+      accumulator.add(new js.Return(exp));
+    } else {
+      accumulator.add(new js.ExpressionStatement(exp));
+      visitStatement(node.next);
+    }
+  }
+
+  bool isNullReturn(tree_ir.Statement node) {
+    return node is tree_ir.Return && isNull(node.value);
+  }
+
+  bool isEndOfMethod(tree_ir.Statement node) {
+    return isNullReturn(node) ||
+           node is tree_ir.Break && isNullReturn(node.target.binding.next);
   }
 
   @override
   void visitIf(tree_ir.If node) {
     js.Expression condition = visitExpression(node.condition);
     int usesBefore = fallthrough.useCount;
+    // Unless the 'else' part ends the method. make sure to terminate any
+    // uncompletable code paths in the 'then' part.
+    emitUnreachableAsReturn.add(!isEndOfMethod(node.elseStatement));
     js.Statement thenBody = buildBodyStatement(node.thenStatement);
+    emitUnreachableAsReturn.removeLast();
     bool thenHasFallthrough = (fallthrough.useCount > usesBefore);
     if (thenHasFallthrough) {
       js.Statement elseBody = buildBodyStatement(node.elseStatement);
@@ -614,7 +644,9 @@ class CodeGenerator extends tree_ir.StatementVisitor
     shortBreak.push(node.next);
     shortContinue.push(node);
     fallthrough.push(node);
+    emitUnreachableAsReturn.add(true);
     js.Statement body = buildBodyStatement(node.body);
+    emitUnreachableAsReturn.removeLast();
     fallthrough.pop();
     shortContinue.pop();
     shortBreak.pop();
@@ -644,7 +676,9 @@ class CodeGenerator extends tree_ir.StatementVisitor
     shortBreak.push(fallthrough.target);
     shortContinue.push(node);
     fallthrough.push(node);
+    emitUnreachableAsReturn.add(true);
     js.Statement jsBody = buildBodyStatement(node.body);
+    emitUnreachableAsReturn.removeLast();
     fallthrough.pop();
     shortContinue.pop();
     if (shortBreak.useCount > 0) {
@@ -683,8 +717,10 @@ class CodeGenerator extends tree_ir.StatementVisitor
 
   @override
   void visitUnreachable(tree_ir.Unreachable node) {
-    // Output nothing.
-    // TODO(asgerf): Emit a throw/return to assist local analysis in the VM?
+    if (emitUnreachableAsReturn.last) {
+      // Emit a return to assist local analysis in the VM.
+      accumulator.add(new js.Return());
+    }
   }
 
   @override