dart2js: Make functions that appear to be unreachable throw.

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 1605 matching lines @@
             visitThen: () {
               closeAndGotoExit(new HReturn(
                   graph.addConstantBool(false, compiler),
                   sourceInformationBuilder
                       .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));
+        return closeFunction();
+      }
+    }
     function.body.accept(this);
     return closeFunction();
   }
 
   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);
@@ skipping 7290 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)));
   }
 }