support resolving .packages in FE

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 a0b9dcd5d447f08e754c0f9cbd0cfda95c1fa469..d65942fc46a39fd320093beb880eb687dda9063b 100644
--- a/pkg/front_end/messages.yaml
+++ b/pkg/front_end/messages.yaml
@@ -790,6 +790,9 @@ MissingMain:
   tip: "Try adding a method named 'main' to your program."
   dart2jsCode: MISSING_MAIN
 
+MissingInputs:
+  template: "No input files provided to the compiler."

[ahe, 2017/07/14 13:44:58]

files -> file.

[Siggi Cherem (dart-lang), 2017/07/14 19:54:06]

Done.

+
 MissingInputFile:
   template: "Input file not found: #string."
 
@@ -846,3 +849,6 @@ AccessError:
 
 PreviousUseOfName:
   template: "Previous use of '#name'."
+
+InvalidPackagesFile:
+  template: "Invalid .packages file '#uri':\n  #string."

[ahe, 2017/07/14 13:44:58]

Invalid is a word that we should avoid. How about:

"Unable to read '.packages' file:\n  #string"

Also, in this case, the uri should be in the location, not the message.

[Siggi Cherem (dart-lang), 2017/07/14 19:54:06]

Done. However, now that I am catching the argument-errors from the package-uri
validation, I have a new error that says "invalid". Suggestions are welcomed.
Done, but let's discuss what are the invariants about location here. I had in
mind that location is a point where they can fix the issue. If the problem is in
the input file, location is the file + offset, if the problem is a flag, we
can't really use the location URI. In this case the error could be the file, or
the flag (pointing to the wrong .packages file), in which case I'm not sure I
want to use the broken uri as the location.

A different example, if an import has a URi that doesn't exist, I believe the
location should point to the import, but the message should mention the URI.