Throw Error on unreachable code rather than a String.

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

 // 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.
 
 part of ssa;
 
 class SsaFunctionCompiler implements FunctionCompiler {
   final SsaCodeGeneratorTask generator;
   final SsaBuilderTask builder;
   final SsaOptimizerTask optimizer;
@@ skipping 1614 matching lines @@
                       .buildImplicitReturn(functionElement)));
             },
             visitElse: null,
             sourceInformation: sourceInformationBuilder.buildIf(function.body));
       }
     }
     if (const bool.fromEnvironment('unreachable-throw') == true) {
       var emptyParameters = parameters.values.where((p) =>
           p.instructionType.isEmpty && !p.instructionType.isNullable);
       if (emptyParameters.length > 0) {
-        String message = compiler.enableMinification
-            ? 'unreachable'
-            : 'unreachable: ${functionElement} because: ${emptyParameters}';
-        // TODO(sra): Use a library function that throws a proper error object.
-        push(new HForeignCode(
-            js.js.parseForeignJS('throw "$message"'),
-            backend.dynamicType,
-            <HInstruction>[],
-            isStatement: true));
+        addComment('${emptyParameters} inferred as [empty]');
+        pushInvokeStatic(function.body, backend.assertUnreachableMethod, []);
+        pop();
         return closeFunction();
       }
     }
     function.body.accept(this);
     return closeFunction();
   }
 
+  /// Adds a JavaScript comment to the output. The comment will be omitted in
+  /// minified mode.  Each line in [text] is preceded with `//` and indented.
+  /// Use sparingly. In order for the comment to be retained it is modeled as
+  /// having side effects which will inhibit code motion.
+  // TODO(sra): Figure out how to keep comment anchored without effects.
+  void addComment(String text) {
+    add(new HForeignCode(
+        js.js.statementTemplateYielding(new js.Comment(text)),
+        backend.dynamicType,
+        <HInstruction>[],
+        isStatement: true));
+  }
+
   HGraph buildCheckedSetter(VariableElement field) {
     openFunction(field, field.node);
     HInstruction thisInstruction = localsHandler.readThis();
     // Use dynamic type because the type computed by the inferrer is
     // narrowed to the type annotation.
     HInstruction parameter = new HParameterValue(field, backend.dynamicType);
     // Add the parameter as the last instruction of the entry block.
     // If the method is intercepted, we want the actual receiver
     // to be the first parameter.
     graph.entry.addBefore(graph.entry.last, parameter);
@@ skipping 7306 matching lines @@
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     JavaScriptBackend backend = builder.compiler.backend;
     ClassElement cls = backend.findHelper('DynamicRuntimeType');
     builder.push(new HDynamicType(type, new TypeMask.exact(cls, classWorld)));
   }
 }