Revert earlier changes to fix bots (TBR)

pkg/analyzer/lib/src/fasta/ast_builder.dart

Index: pkg/analyzer/lib/src/fasta/ast_builder.dart
diff --git a/pkg/analyzer/lib/src/fasta/ast_builder.dart b/pkg/analyzer/lib/src/fasta/ast_builder.dart
index 183f3c19cc22e49f167f2dff0f4d54490f5b38e1..04ae8243d5e8cc417ab5a1a58f4c71730dfb092c 100644
--- a/pkg/analyzer/lib/src/fasta/ast_builder.dart
+++ b/pkg/analyzer/lib/src/fasta/ast_builder.dart
@@ -71,12 +71,6 @@ class AstBuilder extends ScopeListener {
   // * The current library has an import that uses the scheme "dart-ext".
   bool allowNativeClause = false;
 
-  /// A flag indicating whether an exception should be thrown when an error is
-  /// reported for which we have no mapping into an analyzer error.
-  //
-  // TODO(brianwilkerson) Remove this flag after failing tests have been triaged.
-  bool throwOnMissingErrorMapping = true;
-
   AstBuilder(this.errorReporter, this.library, this.member, Scope scope,
       this.isFullAst,
       [Uri uri])
@@ -1087,9 +1081,7 @@ class AstBuilder extends ScopeListener {
         return token;
       }
     }
-    if (throwOnMissingErrorMapping) {
-      throw new UnimplementedError('Failed to map $message at $token');
-    }
+    return super.handleUnrecoverableError(token, message);
   }
 
   void handleUnaryPrefixExpression(Token token) {
@@ -1964,9 +1956,6 @@ class AstBuilder extends ScopeListener {
       default:
       // fall through
     }
-    if (throwOnMissingErrorMapping) {
-      throw new UnimplementedError('Failed to map $message at $charOffset');
-    }
   }
 
   /// A marker method used to mark locations where a token is being located in