Catch file-not-found exception

lib/src/compiler/compiler.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.
 
 import 'dart:collection' show HashSet;
 import 'package:args/args.dart' show ArgParser, ArgResults;
+import 'package:args/src/usage_exception.dart' show UsageException;
 import 'package:analyzer/analyzer.dart'
     show
         AnalysisError,
         CompilationUnit,
         CompileTimeErrorCode,
         ErrorSeverity,
         StaticWarningCode;
 import 'package:analyzer/src/generated/engine.dart' show AnalysisContext;
-import 'package:analyzer/src/generated/java_engine.dart' show AnalysisException;
 import 'package:analyzer/src/generated/source_io.dart' show Source, SourceKind;
 import 'package:func/func.dart' show Func1;
 import 'package:path/path.dart' as path;
 
 import '../analyzer/context.dart'
     show AnalyzerOptions, createAnalysisContextWithSources;
 import 'extension_types.dart' show ExtensionTypeSet;
 import 'code_generator.dart' show CodeGenerator;
 import 'error_helpers.dart' show errorSeverity, formatError, sortErrors;
 
@@ skipping 44 matching lines @@
     // Validate that all parts were explicitly passed in.
     // If not, it's an error.
     var explicitParts = new HashSet<Source>();
     var usedParts = new HashSet<Source>();
     for (var sourcePath in unit.sources) {
       var sourceUri = Uri.parse(sourcePath);
       if (sourceUri.scheme == '') {
         sourceUri = path.toUri(path.absolute(sourcePath));
       }
       Source source = context.sourceFactory.forUri2(sourceUri);
+
+      var fileUsage = 'You need to pass at least one existing .dart file as an'
+          ' argument.';
       if (source == null) {
-        throw new AnalysisException('could not create a source for $sourcePath.'
-            ' The file name is in the wrong format or was not found.');
+        throw new UsageException(
+            'Could not create a source for "$sourcePath". The file name is in'
+            ' the wrong format or was not found.',
+            fileUsage);
+      } else if (!source.exists()) {
+        throw new UsageException(
+            'Given file "$sourcePath" does not exist.', fileUsage);
       }
 
       // Ignore parts. They need to be handled in the context of their library.
       if (context.computeKindOf(source) == SourceKind.PART) {
         explicitParts.add(source);
         continue;
       }
 
       var resolvedTree = context.resolveCompilationUnit2(source, source);
       trees.add(resolvedTree);
@@ skipping 249 matching lines @@
   }
 }
 
 /// (Public for tests) the error code used when a part is missing.
 final missingPartErrorCode = const CompileTimeErrorCode(
     'MISSING_PART', 'The part was not supplied as an input to the compiler.');
 
 /// (Public for tests) the error code used when a part is unused.
 final unusedPartWarningCode = const StaticWarningCode(
     'UNUSED_PART', 'The part was not used by any libraries being compiled.');