Use messages for (some) public API errors

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 604 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."
 
+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:
   template: "Export of '#name' (from '#uri') hides export from '#uri2'."
 
 ImportHidesImport:
   template: "Import of '#name' (from '#uri') hides import from '#uri2'."
 
 DuplicatedExport:
   template: "'#name' is exported from both '#uri' and '#uri2'."
 
 DuplicatedImport:
   template: "'#name' is imported from both '#uri' and '#uri2'."
+
+MissingMain:
+  template: "No 'main' method found."
+  tip: "Try adding a method named 'main' to your program."
+  dart2jsCode: MISSING_MAIN
+
+MissingInputFile:
+  template: "Input file not found: #string."

[ahe, 2017/07/13 00:03:38]

#uri here and below?

[Siggi Cherem (dart-lang), 2017/07/18 00:32:57]

Done.

+
+MissingSdkRoot:
+  template: "SDK root directory not found: #string."
+
+MissingSdkSummary:
+  template: "SDK summary not found: #string."