Only generate coverageIds when needed

pkg/compiler/lib/src/dump_info.dart

Index: pkg/compiler/lib/src/dump_info.dart
diff --git a/pkg/compiler/lib/src/dump_info.dart b/pkg/compiler/lib/src/dump_info.dart
index 07089d8ae99bb15f9709b754f4bd0c850b801045..9f1efbc8824981909919fbab88743c0fdfbdc5d4 100644
--- a/pkg/compiler/lib/src/dump_info.dart
+++ b/pkg/compiler/lib/src/dump_info.dart
@@ -130,9 +130,6 @@ class ElementInfoCollector extends BaseElementVisitor<Info, dynamic> {
 
     FieldInfo info = new FieldInfo(
         name: element.name,
-        // We use element.hashCode because it is globally unique and it is
-        // available while we are doing codegen.
-        coverageId: '${element.hashCode}',
         type: '${element.type}',
         inferredType: '$inferredType',
         code: code,
@@ -147,6 +144,12 @@ class ElementInfoCollector extends BaseElementVisitor<Info, dynamic> {
       }
     }
 
+    if (JavaScriptBackend.TRACE_METHOD == 'post') {
+      // We use element.hashCode because it is globally unique and it is
+      // available while we are doing codegen.
+      info.coverageId = '${element.hashCode}';
+    }
+
     int closureSize = _addClosureInfo(info, element);
     info.size = size + closureSize;
 
@@ -274,9 +277,6 @@ class ElementInfoCollector extends BaseElementVisitor<Info, dynamic> {
     FunctionInfo info = new FunctionInfo(
         name: name,
         functionKind: kind,
-        // We use element.hashCode because it is globally unique and it is
-        // available while we are doing codegen.
-        coverageId: '${element.hashCode}',
         modifiers: modifiers,
         returnType: returnType,
         inferredReturnType: inferredReturnType,
@@ -295,6 +295,12 @@ class ElementInfoCollector extends BaseElementVisitor<Info, dynamic> {
       info.closures = <ClosureInfo>[];
     }
 
+    if (JavaScriptBackend.TRACE_METHOD == 'post') {
+      // We use element.hashCode because it is globally unique and it is
+      // available while we are doing codegen.
+      info.coverageId = '${element.hashCode}';
+    }
+
     info.size = size;
 
     result.functions.add(info);