Return empty contents for not existing sources.

pkg/analyzer/lib/src/context/cache.dart

Index: pkg/analyzer/lib/src/context/cache.dart
diff --git a/pkg/analyzer/lib/src/context/cache.dart b/pkg/analyzer/lib/src/context/cache.dart
index 4c4c46fdf594dcd1f074f1f0f529d56935ea2a5d..ded58c9b5c44dbd1e13a218b6ab6a99220122286 100644
--- a/pkg/analyzer/lib/src/context/cache.dart
+++ b/pkg/analyzer/lib/src/context/cache.dart
@@ -225,9 +225,9 @@ class CacheEntry {
 
   /**
    * The most recent time at which the state of the target matched the state
-   * represented by this entry.
+   * represented by this entry, `-1` if the target does not exist.
    */
-  int modificationTime = 0;
+  int modificationTime = -1;
 
   /**
    * The exception that caused one or more values to have a state of