Fix an exception caused by incomplete input (issue 29687)

pkg/analyzer/lib/src/summary/summarize_const_expr.dart

Index: pkg/analyzer/lib/src/summary/summarize_const_expr.dart
diff --git a/pkg/analyzer/lib/src/summary/summarize_const_expr.dart b/pkg/analyzer/lib/src/summary/summarize_const_expr.dart
index 6212f5d5d76e1f505e2872f71143f16807e545e7..85ff646bdeef1680e63299bc1bfec6b1fb33d078 100644
--- a/pkg/analyzer/lib/src/summary/summarize_const_expr.dart
+++ b/pkg/analyzer/lib/src/summary/summarize_const_expr.dart
@@ -286,6 +286,9 @@ abstract class AbstractConstExprSerializer {
   }
 
   void _pushInt(int value) {
+    if (value == null) {
+      value = 0;

[scheglov, 2017/05/30 17:25:18]

This could be also.

value ??= 0;

[Brian Wilkerson, 2017/05/30 17:38:45]

Done

+    }
     assert(value >= 0);
     if (value >= 0x100000000) {
       int numOfComponents = 0;