Switch FE to use the libraries.json format.

pkg/front_end/messages.yaml

Index: pkg/front_end/messages.yaml
diff --git a/pkg/front_end/messages.yaml b/pkg/front_end/messages.yaml
index b0a3d1c618dcf5a985b8518df87e9c5d50f2c21c..ca0c8d66efe053cef46cd53300d0e8463dd37404 100644
--- a/pkg/front_end/messages.yaml
+++ b/pkg/front_end/messages.yaml
@@ -823,6 +823,10 @@ SdkRootNotFound:
 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'."
 
@@ -883,6 +887,9 @@ NotAnLvalue:
 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."