dart2js: fail gracefully with invalid package config

Fixes #24118
Fixes #24120
Fixes #24121
Fixes #24122

BUG=
[email protected]

Review URL: https://codereview.chromium.org//1291283006 .

Author: harryterkelsen

pkg/compiler/lib/src/apiimpl.dart

@@ -381,13 +381,17 @@ class Compiler extends leg.Compiler {
         }
         // The input provider may put a trailing 0 byte when it reads a source
         // file, which confuses the package config parser.
-        if (packageConfigContents.length > 0
-            && packageConfigContents.last == 0) {
+        if (packageConfigContents.length > 0 &&
+            packageConfigContents.last == 0) {
           packageConfigContents = packageConfigContents.sublist(
               0, packageConfigContents.length - 1);
         }
         packages =
             new MapPackages(pkgs.parse(packageConfigContents, packageConfig));
+      }).catchError((error) {
+        reportError(NO_LOCATION_SPANNABLE, MessageKind.INVALID_PACKAGE_CONFIG,
+            {'uri': packageConfig, 'exception': error});
+        packages = Packages.noPackages;
       });
     } else {
       if (packagesDiscoveryProvider == null) {

pkg/compiler/lib/src/warnings.dart

@@ -252,6 +252,7 @@ enum MessageKind {
   INVALID_OVERRIDE_METHOD,
   INVALID_OVERRIDE_SETTER,
   INVALID_OVERRIDE_SETTER_WITH_FIELD,
+  INVALID_PACKAGE_CONFIG,
   INVALID_PACKAGE_URI,
   INVALID_PARAMETER,
   INVALID_RECEIVER_IN_INITIALIZER,
@@ -2224,6 +2225,13 @@ import '../../Udyn[mic ils/expect.dart';
 main() {}
 """]),
 
+      MessageKind.INVALID_PACKAGE_CONFIG:
+          const MessageTemplate(MessageKind.INVALID_PACKAGE_CONFIG,
+            """Package config file '#{uri}' is invalid.
+#{exception}""",
+            howToFix: DONT_KNOW_HOW_TO_FIX
+      ),
+
       MessageKind.INVALID_PACKAGE_URI:
         const MessageTemplate(MessageKind.INVALID_PACKAGE_URI,
           "'#{uri}' is not a valid package URI (#{exception}).",