Produce a compile-time error if there's no main method.

pkg/front_end/lib/src/fasta/fasta.dart

 // Copyright (c) 2016, 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.
 
 library fasta;
 
 import 'dart:async' show Future;
 
 import 'dart:convert' show JSON;
 
@@ skipping 167 matching lines @@
       program = await kernelTarget.writeProgram(null);
       if (kernelTarget.errors.isNotEmpty) {
         return new CompilationResult.errors(kernelTarget.errors
             .map((err) => err.toString())
             .toList(growable: false));
       }
     } on InputError catch (e) {
       return new CompilationResult.error(e.format());
     }
 
+    if (program.mainMethod == null) {
+      return new CompilationResult.error("No 'main' method found.");
+    }
+
     // Perform target-specific transformations.
     target.performModularTransformations(program);
     target.performGlobalTransformations(program);
 
     // Write the program to a list of bytes and return it.
     var sink = new ByteSink();
     new BinaryPrinter(sink).writeProgramFile(program);
     return new CompilationResult.ok(sink.builder.takeBytes());
   } catch (e, s) {
     return reportCrash(e, s, fileName);
@@ skipping 46 matching lines @@
   final BytesBuilder builder = new BytesBuilder();
 
   void add(List<int> data) {
     builder.add(data);
   }
 
   void close() {
     // Nothing to do.
   }
 }