Add a draft of traversing visitor which will analyse function.

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
new file mode 100644
index 0000000000000000000000000000000000000000..a7beccf9b31163e01aa369a16265ca5726af570a
--- /dev/null
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -0,0 +1,102 @@
+// Copyright (c) 2012, 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.
+
+/**
+ * Visitor to gather conservative estimate of functions which
+ * can be invoked and classes which can be instantiated.
+ */
+class ReachabilityVisitor implements Visitor {
+  visitBlock(Block node) {}
+  visitBreakStatement(BreakStatement node) {}
+  visitCascade(Cascade node) {}
+  visitCascadeReceiver(CascadeReceiver node) {}
+  visitCaseMatch(CaseMatch node) {}
+  visitCatchBlock(CatchBlock node) {}
+  visitClassNode(ClassNode node) {
+    internalError('should not reach ClassNode');
+  }
+  visitConditional(Conditional node) {}
+  visitContinueStatement(ContinueStatement node) {}
+  visitDoWhile(DoWhile node) {}
+  visitEmptyStatement(EmptyStatement node) {}
+  visitExpressionStatement(ExpressionStatement node) {}
+  visitFor(For node) {}
+  visitForIn(ForIn node) {}
+  visitFunctionDeclaration(FunctionDeclaration node) {}
+  visitFunctionExpression(FunctionExpression node) {
+    // TODO(antonm): add a closure to working queue.
+    unimplemented('FunctionExpression is not supported');
+  }
+  visitIdentifier(Identifier node) {}
+  visitIf(If node) {}
+  visitLabel(Label node) {}
+  visitLabeledStatement(LabeledStatement node) {}
+  visitLiteralBool(LiteralBool node) {}
+  visitLiteralDouble(LiteralDouble node) {}
+  visitLiteralInt(LiteralInt node) {}
+  visitLiteralList(LiteralList node) {}
+  visitLiteralMap(LiteralMap node) {}
+  visitLiteralMapEntry(LiteralMapEntry node) {}
+  visitLiteralNull(LiteralNull node) {}
+  visitLiteralString(LiteralString node) {}
+  visitModifiers(Modifiers node) {}
+  visitNamedArgument(NamedArgument node) {}
+  visitNewExpression(NewExpression node) {
+    // TODO(antonm): update set of instantiated classes.
+    unimplemented('NewExpression is not supported');
+  }
+  visitNodeList(NodeList node) {}
+  visitOperator(Operator node) {}
+  visitParenthesizedExpression(ParenthesizedExpression node) {}
+  visitReturn(Return node) {}
+  visitScriptTag(ScriptTag node) {}
+  visitSend(Send node) {
+    // TODO(antonm): update working queue.
+    unimplemented('Send is not supported');
+  }
+  visitSendSet(SendSet node) {
+    // TODO(antonm): update working queue.
+    unimplemented('SendSet is not supported');
+  }
+  visitStringInterpolation(StringInterpolation node) {}
+  visitStringInterpolationPart(StringInterpolationPart node) {}
+  visitStringJuxtaposition(StringJuxtaposition node) {}
+  visitSwitchCase(SwitchCase node) {}
+  visitSwitchStatement(SwitchStatement node) {}
+  visitThrow(Throw node) {}
+  visitTryStatement(TryStatement node) {}
+  visitTypeAnnotation(TypeAnnotation node) {}
+  visitTypedef(Typedef node) {}
+  visitTypeVariable(TypeVariable node) {}
+  visitVariableDefinitions(VariableDefinitions node) {}
+  visitWhile(While node) {}
+
+  unimplemented(String reason) {
+    throw new CompilerCancelledException('not implemented: $reason');
+  }
+
+  internalError(String reason) {
+    throw new CompilerCancelledException('internal error: $reason');
+  }
+}
+
+class DartBackend extends Backend {
+  final ReachabilityVisitor reachabilityVisitor;
+  final List<CompilerTask> tasks = const <CompilerTask>[];
+
+  DartBackend(Compiler compiler)
+      : reachabilityVisitor = new ReachabilityVisitor(),
+        super(compiler);
+
+  String codegen(WorkItem work) {
+    // Traverse AST to populate sets of reachable classes and functions.
+    FunctionExpression function = work.element.parseNode(compiler);
+    function.body.accept(new TraversingVisitor(reachabilityVisitor));
+  }
+
+  void processNativeClasses(world, libraries) {}
+  void assembleProgram() {
+    compiler.assembledCode = '';
+  }
+}