Report internal error when an exported library isn't loaded.

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 642 matching lines @@
 InternalProblemMissingContext:
   template: "Compiler cannot run without a compiler context."
   tip: "Are calls to the compiler wrapped in CompilerContext.runInContext?"
 
 InternalProblemProvidedBothCompileSdkAndSdkSummary:
   template: "The compileSdk and sdkSummary options are mutually exclusive"
 
 InternalProblemUriMissingScheme:
   template: "The URI '#uri' has no scheme."
 
+InternalProblemExportedLibraryNotLoaded:
+  template: "The library '#uri' isn't loaded, but '#uri2' exports it."
+
 InternalVerificationError:
   template: "Verification of the generated program failed: #string."
 
 LocalDefinitionHidesExport:
   template: "Local definition of '#name' hides export from '#uri'."
 
 LocalDefinitionHidesImport:
   template: "Local definition of '#name' hides import from '#uri'."
 
 ExportHidesExport:
@@ skipping 255 matching lines @@
 PackageNotFound:
   template: "Could not resolve the package '#name' in '#uri'."
 
 InvalidPackageUri:
   template: "Invalid package Uri '#uri':\n  #string."
 
 IntegerLiteralIsOutOfRange:
   template: "The integer literal #lexeme can't be represented in 64 bits."
   tip: "Try using BigInt (from 'dart:typed_data' library) if you need an integer larger than 9,223,372,036,854,775,807 or less than -9,223,372,036,854,775,808."