Implement try/catch in the JS backend.

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

Index: pkg/compiler/lib/src/js_backend/codegen/unsugar.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/unsugar.dart b/pkg/compiler/lib/src/js_backend/codegen/unsugar.dart
index a071b47a3b16a5fe29305fb9f578dfc0c252f040..4a8ee1fee234ae4c64df3578ccdbc4b80f1e76b4 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/unsugar.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/unsugar.dart
@@ -6,7 +6,8 @@ import '../../cps_ir/cps_ir_nodes.dart';
 import '../../cps_ir/optimizers.dart';
 import '../../constants/expressions.dart';
 import '../../constants/values.dart';
-import '../../elements/elements.dart' show ClassElement, FieldElement, Element;
+import '../../elements/elements.dart'
+    show ClassElement, FieldElement, FunctionElement, Element;
 import '../../js_backend/codegen/glue.dart';
 import '../../dart2jslib.dart' show Selector, World;
 
@@ -15,14 +16,16 @@ import '../../dart2jslib.dart' show Selector, World;
 ///
 /// Performs the following rewrites:
 ///  - rewrite [IsTrue] in a [Branch] to do boolean conversion.
+///  - converts two-parameter exception handlers to one-parameter ones.
 class UnsugarVisitor extends RecursiveVisitor {
   Glue _glue;
+  ParentVisitor _parentVisitor = new ParentVisitor();
 
   UnsugarVisitor(this._glue);
 
   void rewrite(FunctionDefinition function) {
     // Set all parent pointers.
-    new ParentVisitor().visit(function);
+    _parentVisitor.visit(function);
     visit(function);
   }
 
@@ -47,6 +50,62 @@ class UnsugarVisitor extends RecursiveVisitor {
     let.parent = parent;
   }
 
+  /// Insert a static call to [function] at the point of [node] with result
+  /// [result].
+  ///
+  /// Rewrite [node] to
+  ///
+  /// let cont continuation(result) = node
+  /// in invoke function arguments continuation
+  void insertStaticCall(FunctionElement function, List<Primitive> arguments,
+      Parameter result,
+      Expression node) {
+    InteriorNode parent = node.parent;
+    Continuation continuation = new Continuation([result]);
+    continuation.body = node;
+    _parentVisitor.processContinuation(continuation);
+
+    Selector selector = new Selector.fromElement(function);
+    // TODO(johnniwinther): Come up with an implementation of SourceInformation
+    // for calls such as this one that don't appear in the original source.
+    InvokeStatic invoke =
+        new InvokeStatic(function, selector, continuation, arguments, null);
+    _parentVisitor.processInvokeStatic(invoke);
+
+    LetCont letCont = new LetCont(continuation, invoke);
+    _parentVisitor.processLetCont(letCont);
+
+    parent.body = letCont;
+    letCont.parent = parent;
+  }
+
+  processLetHandler(LetHandler node) {
+    // BEFORE: Handlers have two parameters, exception and stack trace.
+    // AFTER: Handlers have a single parameter, which is unwrapped to get
+    // the exception and stack trace.
+    assert(node.handler.parameters.length == 2);
+    Parameter exceptionParameter = node.handler.parameters.first;
+    Parameter stackTraceParameter = node.handler.parameters.last;
+    Expression body = node.handler.body;
+    if (exceptionParameter.hasAtLeastOneUse ||
+        stackTraceParameter.hasAtLeastOneUse) {
+      Parameter exceptionValue = new Parameter(null);
+      exceptionValue.substituteFor(exceptionParameter);
+      insertStaticCall(_glue.getExceptionUnwrapper(), [exceptionParameter],
+          exceptionValue, body);
+
+      if (stackTraceParameter.hasAtLeastOneUse) {
+        Parameter stackTraceValue = new Parameter(null);
+        stackTraceValue.substituteFor(stackTraceParameter);
+        insertStaticCall(_glue.getTraceFromException(), [exceptionValue],
+            stackTraceValue, body);
+      }
+    }
+
+    assert(stackTraceParameter.hasNoUses);
+    node.handler.parameters.removeLast();
+  }
+
   processInvokeMethod(InvokeMethod node) {
     Selector selector = node.selector;
     // TODO(karlklose):  should we rewrite all selectors?