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

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

Index: pkg/front_end/lib/src/fasta/fasta.dart
diff --git a/pkg/front_end/lib/src/fasta/fasta.dart b/pkg/front_end/lib/src/fasta/fasta.dart
index ba3161808eb2e597fe0d894f5833f0ad0903d740..53da342394a6db6f115438e803202299f60094bf 100644
--- a/pkg/front_end/lib/src/fasta/fasta.dart
+++ b/pkg/front_end/lib/src/fasta/fasta.dart
@@ -185,6 +185,10 @@ Future<CompilationResult> parseScript(
       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);