Disable source maps completely.

dart/pkg/compiler/lib/src/js_backend/backend.dart

 // Copyright (c) 2012, 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.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 const bool USE_CPS_IR = const bool.fromEnvironment("USE_CPS_IR");
 
@@ skipping 443 matching lines @@
         rti = new RuntimeTypes(compiler),
         specializedGetInterceptors = new Map<String, Set<ClassElement>>(),
         super(compiler) {
     emitter = new CodeEmitterTask(compiler, namer, generateSourceMap);
     typeVariableHandler = new TypeVariableHandler(this);
     customElementsAnalysis = new CustomElementsAnalysis(this);
     constantCompilerTask = new JavaScriptConstantTask(compiler);
     resolutionCallbacks = new JavaScriptResolutionCallbacks(this);
     functionCompiler = USE_CPS_IR
          ? new CspFunctionCompiler(compiler, this)
-         : new SsaFunctionCompiler(this);
+         : new SsaFunctionCompiler(this, generateSourceMap);
   }
 
   ConstantSystem get constantSystem => constants.constantSystem;
 
   /// Returns constant environment for the JavaScript interpretation of the
   /// constants.
   JavaScriptConstantCompiler get constants {
     return constantCompilerTask.jsConstantCompiler;
   }
 
@@ skipping 1994 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }