Fix invariant breakage on erroneous constant variables.

pkg/compiler/lib/src/constants/expressions.dart

Index: pkg/compiler/lib/src/constants/expressions.dart
diff --git a/pkg/compiler/lib/src/constants/expressions.dart b/pkg/compiler/lib/src/constants/expressions.dart
index aa284cd20ceb174fe169a5ec994c99f4c2c0eed2..a9735065e098ea84d3adba28515d12d208df9be7 100644
--- a/pkg/compiler/lib/src/constants/expressions.dart
+++ b/pkg/compiler/lib/src/constants/expressions.dart
@@ -1261,7 +1261,11 @@ class BoolFromEnvironmentConstantExpression
     sb.write('bool.fromEnvironment(name=');
     name._createStructuredText(sb);
     sb.write(',defaultValue=');
-    defaultValue._createStructuredText(sb);
+    if (defaultValue != null) {
+      defaultValue._createStructuredText(sb);
+    } else {
+      sb.write('null');
+    }
     sb.write(')');
   }
 
@@ -1320,7 +1324,11 @@ class IntFromEnvironmentConstantExpression
     sb.write('int.fromEnvironment(name=');
     name._createStructuredText(sb);
     sb.write(',defaultValue=');
-    defaultValue._createStructuredText(sb);
+    if (defaultValue != null) {
+      defaultValue._createStructuredText(sb);
+    } else {
+      sb.write('null');
+    }
     sb.write(')');
   }
 
@@ -1381,7 +1389,11 @@ class StringFromEnvironmentConstantExpression
     sb.write('String.fromEnvironment(name=');
     name._createStructuredText(sb);
     sb.write(',defaultValue=');
-    defaultValue._createStructuredText(sb);
+    if (defaultValue != null) {
+      defaultValue._createStructuredText(sb);
+    } else {
+      sb.write('null');
+    }
     sb.write(')');
   }