Throw if there is no compiler context available

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

Index: pkg/front_end/lib/src/fasta/compiler_command_line.dart
diff --git a/pkg/front_end/lib/src/fasta/compiler_command_line.dart b/pkg/front_end/lib/src/fasta/compiler_command_line.dart
index 345a304d3c6269edfe2ed6908cb37ca74161310f..f3c60fd7def66217ec71df225492f439dbc52fb2 100644
--- a/pkg/front_end/lib/src/fasta/compiler_command_line.dart
+++ b/pkg/front_end/lib/src/fasta/compiler_command_line.dart
@@ -156,10 +156,6 @@ class CompilerCommandLine extends CommandLine {
     return CompilerContext.withGlobalOptions(
         new CompilerCommandLine(programName, arguments), f);
   }
-
-  static CompilerCommandLine forRootContext() {
-    return new CompilerCommandLine("", [""]);
-  }
 }
 
 Message computeUsage(String programName, bool verbose) {