Throw if there is no compiler context available

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 610 matching lines @@
 
 InternalProblemStackNotEmpty:
   template: "#name.stack isn't empty:\n  #string"
 
 InternalProblemAlreadyInitialized:
   template: "Attempt to set initializer on field without initializer."
 
 InternalProblemBodyOnAbstractMethod:
   template: "Attempting to set body on abstract method."
 
+InternalProblemMissingContext:
+  template: "Compiler cannot run without a compiler context."
+  tip: "Wrap calls to the compiler in a zone with a compiler context."

[ahe, 2017/07/17 08:42:07]

This sounds like an order (which is impolite). Also, it isn't obvious how to
create that zone. How about:

Are all calls to the compiler are wrapped in
'CompilerCommandLine.withGlobalOptions'?

This is the non-commanding version of "Make sure all calls...".

[Siggi Cherem (dart-lang), 2017/07/17 20:18:38]

Done.

+
 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 227 matching lines @@
 
 CantInferPackagesFromManyInputs:
   template: "Cannot infer a .packages file when compiling multiple inputs."
   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."