Don't create FileState(s) for SDK source in _ContentCacheWrapper.

pkg/analyzer/lib/src/dart/analysis/driver.dart

Index: pkg/analyzer/lib/src/dart/analysis/driver.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index 0d16a9221cc11a1210f6ff601aecc10090dfc68a..b5ea6e8f1cb685ef8d918c5f730bdefb6712e011 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -1549,11 +1549,17 @@ class _ContentCacheWrapper implements ContentCache {
 
   @override
   bool getExists(Source source) {
+    if (source.isInSystemLibrary) {
+      return true;
+    }
     return _getFileForSource(source).exists;
   }
 
   @override
   int getModificationStamp(Source source) {
+    if (source.isInSystemLibrary) {
+      return 0;
+    }
     return _getFileForSource(source).exists ? 0 : -1;
   }