Propagate bailout messages from the CPS IR backends.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index 6541e41a2533146b0cbf615a0c3a7ffff500457e..a0b91ef7987fc1b46820c1f7a0b2ad9b819cade5 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -45,6 +45,8 @@ class IrBuilderTask extends CompilerTask {
       <Element, ir.ExecutableDefinition>{};
   final bool generateSourceMap;
 
+  String bailoutMessage = null;
+
   IrBuilderTask(Compiler compiler, {this.generateSourceMap: true})
       : super(compiler);
 
@@ -57,7 +59,11 @@ class IrBuilderTask extends CompilerTask {
   }
 
   ir.ExecutableDefinition buildNode(AstElement element) {
-    if (!canBuild(element)) return null;
+    bailoutMessage = null;
+    if (!canBuild(element)) {
+      bailoutMessage = 'unsupported element ${element.name}:${element.kind}';
+      return null;
+    }
 
     TreeElements elementsMapping = element.resolvedAst.elements;
     element = element.implementation;
@@ -74,7 +80,9 @@ class IrBuilderTask extends CompilerTask {
               elementsMapping, compiler, sourceInformationBuilder);
       ir.ExecutableDefinition definition =
             builder.buildExecutable(element);
-      if (definition != null) {
+      if (definition == null) {
+        bailoutMessage = builder.bailoutMessage;
+      } else {
         nodes[element] = definition;
       }
       return definition;
@@ -163,6 +171,8 @@ abstract class IrBuilderVisitor extends SemanticVisitor<ir.Primitive, dynamic>
   @override
   bulkHandleNode(ast.Node node, String message, _) => giveup(node, message);
 
+  String bailoutMessage = null;
+
   @override
   ir.Primitive apply(ast.Node node, _) => node.accept(this);
 
@@ -1761,21 +1771,22 @@ abstract class IrBuilderVisitor extends SemanticVisitor<ir.Primitive, dynamic>
   }
 
   void internalError(ast.Node node, String message) {
-    giveup(node);
+    giveup(node, message);
   }
 
   @override
   visitNode(ast.Node node) {
     internalError(node, "Unhandled node");
   }
+
+  dynamic giveup(ast.Node node, [String reason]) {
+    bailoutMessage = '($node): $reason';
+    throw ABORT_IRNODE_BUILDER;
+  }
 }
 
 final String ABORT_IRNODE_BUILDER = "IrNode builder aborted";
 
-dynamic giveup(ast.Node node, [String reason]) {
-  throw ABORT_IRNODE_BUILDER;
-}
-
 /// Classifies local variables and local functions as captured, if they
 /// are accessed from within a nested function.
 ///
@@ -1797,6 +1808,13 @@ class DartCapturedVariables extends ast.Visitor {
   List<TryStatementInfo> tryNestingStack = <TryStatementInfo>[];
   bool get inTryStatement => tryNestingStack.isNotEmpty;
 
+  String bailoutMessage = null;
+
+  giveup(ast.Node node, [String reason]) {
+    bailoutMessage = '($node): $reason';
+    throw ABORT_IRNODE_BUILDER;
+  }
+
   void markAsCaptured(Local local) {
     capturedVariables.add(local);
   }
@@ -1929,7 +1947,12 @@ class DartIrBuilderVisitor extends IrBuilderVisitor {
   DartIrBuilder makeIRBuilder(ast.Node node, ExecutableElement element) {
     DartCapturedVariables closures = new DartCapturedVariables(elements);
     if (!element.isSynthesized) {
-      closures.visit(node);
+      try {
+        closures.visit(node);
+      } catch (e) {
+        bailoutMessage = closures.bailoutMessage;
+        rethrow;
+      }
     }
     return new DartIrBuilder(compiler.backend.constantSystem,
                              element,