Always use the same json for null in the vm service.

runtime/bin/vmservice/client/deployed/web/index_devtools.html_bootstrap.dart.js

 // Generated by dart2js, the Dart to JavaScript compiler.
 (function($){function dart(){this.x=0}var A=new dart
 delete A.x
 var B=new dart
 delete B.x
 var C=new dart
 delete C.x
 var D=new dart
 delete D.x
 var E=new dart
@@ skipping 2384 matching lines @@
 "^":"Tp:300;a",
 $2:[function(a,b){var z,y,x,w
 z=this.a
 y=z.zU
 if(a>>>0!==a||a>=y.length)return H.e(y,a)
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 $.uP=!1
 F.E2()},"$0","KU",0,0,125]},1],["breakpoint_list_element","package:observatory/src/elements/breakpoint_list.dart",,B,{
 "^":"",
 pz:{
 "^":["pv;BW%-327,AP,fn,AP,fn,dZ,Sa,Uk,oq,Wz,SO,B7,X0-328",null,null,null,null,null,null,null,null,null,null,null,null,function(){return[C.Nw]}],
 grs:[function(a){return a.BW},null,null,1,0,329,"msg",307,330],
 srs:[function(a,b){a.BW=this.ct(a,C.UX,a.BW,b)},null,null,3,0,331,30,[],"msg",307],
 pA:[function(a,b){J.am(a.BW).YM(b)},"$1","gvC",2,0,169,332,[],"refresh"],
@@ skipping 6943 matching lines @@
 di:function(a){return this.EV.di(a)},
 aN:function(a,b){this.EV.aN(0,b)},
 gl0:function(a){return this.EV.X5===0},
 gor:function(a){return this.EV.X5!==0},
 gvc:function(){var z=this.EV
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 gB:function(a){return this.EV.X5},
 Rz:function(a,b){return this.EV.Rz(0,b)},
 gUQ:function(a){var z=this.EV
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+bu:function(a){return P.vW(this.EV)},
 $isZ0:true}}],["dart.typed_data.implementation","dart:_native_typed_data",,H,{
 "^":"",
 UI:function(a){a.toString
 return a},
 bu:function(a){a.toString
 return a},
 aR:function(a){a.toString
 return a},
 WZ:{
 "^":"Gv;",
@@ skipping 1726 matching lines @@
 if(typeof z!=="number")return H.s(z)
 return this.P-z},
 giO:function(a){return this.P},
 bu:function(a){return this.oc},
 $isqV:true,
 static:{"^":"K9,tmj,EL,LkO,reI,pd,dc,MHK,ow,lM,B9"}},
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 bu:function(a){return"["+this.OR.oc+"] "+this.iJ+": "+this.G1},
 $isHV:true,
-static:{"^":"xO"}}}],["","file:///usr/local/google/home/johnmccutchan/workspace/dart-repo/dart/runtime/bin/vmservice/client/web/main.dart",,F,{
+static:{"^":"xO"}}}],["","file:///Users/turnidge/ws/dart-repo/dart/runtime/bin/vmservice/client/web/main.dart",,F,{
 "^":"",
 E2:[function(){N.Jx("").sOR(C.IF)
 N.Jx("").gSZ().yI(new F.em())
 N.Jx("").To("Starting Observatory")
 var z=H.VM(new P.Zf(P.Dt(null)),[null])
 N.Jx("").To("Loading Google Charts API")
 J.UQ($.cM(),"google").V7("load",["visualization","1",P.jT(P.EF(["packages",["corechart","table"],"callback",new P.r7(P.xZ(z.gv6(z),!0))],null,null))])
 z.MM.ml(G.vN()).ml(new F.Lb())},"$0","qg",0,0,null],
 em:{
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 Lb:{
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 $1:[function(a){N.Jx("").To("Initializing Polymer")
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+$isEH:true}}],["metadata","file:///Users/turnidge/ws/dart-repo/dart/runtime/bin/vmservice/client/web/packages/$sdk/lib/html/html_common/metadata.dart",,B,{
 "^":"",
 jh:{
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 static:{"^":"n4I,ziq,pjg,nq,xa"}},
 tzK:{
 "^":"a;"},
 jA:{
 "^":"a;oc>"},
 PO:{
 "^":"a;"},
@@ skipping 6604 matching lines @@
 function Isolate(){var x=Object.prototype.hasOwnProperty
 for(var w in y)if(x.call(y,w))this[w]=y[w]
 function ForceEfficientMap(){}ForceEfficientMap.prototype=this
 new ForceEfficientMap()}Isolate.prototype=a.prototype
 Isolate.prototype.constructor=Isolate
 Isolate.p=y
 Isolate.$finishClasses=a.$finishClasses
 Isolate.makeConstantList=a.makeConstantList
 return Isolate}}
 })()