Revert "Switch FE to use the libraries.json format."

pkg/front_end/messages.yaml

 # Copyright (c) 2017, 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.
 
 # Each entry in this map corresponds to a diagnostic message. Ideally, each
 # entry contains three parts:
 #
 # 1. A message template (template).
 #
 # 2. A suggestion for how to correct the problem (tip).
@@ skipping 805 matching lines @@
 
 InputFileNotFound:
   template: "Input file not found: #uri."
 
 SdkRootNotFound:
   template: "SDK root directory not found: #uri."
 
 SdkSummaryNotFound:
   template: "SDK summary not found: #uri."
 
-SdkSpecificationNotFound:
-  template: "SDK libraries specification not found: #uri."
-  tip: "Normally, the specification is a file named 'libraries.json' in the Dart SDK install location."
-
 ThisAccessInFieldInitializer:
   template: "Can't access 'this' in a field initializer to read '#name'."
 
 ThisAsIdentifier:
   template: "Expected identifier, but got 'this'."
 
 SuperAsIdentifier:
   template: "Expected identifier, but got 'super'."
 
 SwitchCaseFallThrough:
@@ skipping 40 matching lines @@
 
 ExpectedAnInitializer:
   template: "Expected an initializer."
 
 NotAnLvalue:
   template: "Can't assign to this."
 
 CannotReadPackagesFile:
   template: "Unable to read '.packages' file:\n  #string."
 
-CannotReadSdkSpecification:
-  template: "Unable to read the 'libraries.json' specification file:\n  #string."
-
 CantInferPackagesFromManyInputs:
   template: "Can't infer a .packages file when compiling multiple inputs."
   tip: "Try specifying the file explicitly with the --packages option."
 
 CantInferPackagesFromPackageUri:
   template: "Can't infer a .packages file from an input 'package:*' URI."
   tip: "Try specifying the file explicitly with the --packages option."
 
 PackageNotFound:
   template: "Could not resolve the package '#name' in '#uri'."
 
 InvalidPackageUri:
   template: "Invalid package Uri '#uri':\n  #string."