Handle missing sources properly when building AST-based summaries.

pkg/analyzer_cli/lib/src/build_mode.dart

 // Copyright (c) 2015, 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 analyzer_cli.src.build_mode;
 
 import 'dart:core' hide Resource;
 import 'dart:io' as io;
 
 import 'package:analyzer/dart/ast/ast.dart' show CompilationUnit;
@@ skipping 306 matching lines @@
       // Maybe an input package contains the source.
       {
         UnlinkedUnit unlinkedUnit = summaryDataStore.unlinkedMap[absoluteUri];
         if (unlinkedUnit != null) {
           return unlinkedUnit;
         }
       }
       // Parse the source and serialize its AST.
       Uri uri = Uri.parse(absoluteUri);
       Source source = context.sourceFactory.forUri2(uri);
+      if (!source.exists()) {
+        // TODO(paulberry): we should report a warning/error because DDC
+        // compilations are unlikely to work.
+        return null;
+      }
       return uriToUnit.putIfAbsent(uri, () {
         CompilationUnit unit = context.computeResult(source, PARSED_UNIT);
         UnlinkedUnitBuilder unlinkedUnit = serializeAstUnlinked(unit);
         assembler.addUnlinkedUnit(source, unlinkedUnit);
         return unlinkedUnit;
       });
     }
 
     Map<String, LinkedLibraryBuilder> linkResult =
         link(sourceUris, _getDependency, _getUnit, options.strongMode);
@@ skipping 15 matching lines @@
             'Illegal input file (must be "\$uri|\$path"): $sourceFile');
         return null;
       }
       Uri uri = Uri.parse(sourceFile.substring(0, pipeIndex));
       String path = sourceFile.substring(pipeIndex + 1);
       uriToFileMap[uri] = new JavaFile(path);
     }
     return uriToFileMap;
   }
 }