Change how we provide value previews in the vm service.

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

 // Generated by dart2js, the Dart to JavaScript compiler.
 (function($){function dart() {}var A=new dart
 delete A.x
 var B=new dart
 delete B.x
 var C=new dart
 delete C.x
 var D=new dart
 delete D.x
 var E=new dart
@@ skipping 10540 matching lines @@
 a.B7=y
 a.X0=w
 C.cp.ZL(a)
 C.cp.oX(a)
 return a},null,null,0,0,110,"new InstanceRefElement$created"]}},
 "+InstanceRefElement":[384],
 Js:{
 "^":"Tp:107;a-77",
 call$1:[function(a){var z,y
 z=J.U6(a)
-if(z.t(a,"preview")!=null){z.soc(a,z.t(a,"preview"))
-a.szz(z.t(a,"preview"))}z=this.a
+if(z.t(a,"valueAsString")!=null){z.soc(a,z.t(a,"valueAsString"))
+a.szz(z.t(a,"valueAsString"))}z=this.a
 y=J.RE(z)
 y.stY(z,y.ct(z,C.kY,y.gtY(z),a))
 y.ct(z,C.kY,0,1)},"call$1",null,2,0,107,51,[],"call"],
 $isEH:true},
 "+InstanceRefElement_expandEvent_closure":[493]}],["instance_view_element","package:observatory/src/elements/instance_view.dart",,Z,{
 "^":"",
 hx:{
 "^":["V0;Xh%-375,AP,fn,AP,fn,dZ,Sa,Uk,oq,Wz,SO,B7,X0-376",null,null,null,null,null,null,null,null,null,null,null,null,function(){return[C.Nw]}],
 gQr:[function(a){return a.Xh},null,null,1,0,377,"instance",358,378],
 sQr:[function(a,b){a.Xh=this.ct(a,C.fn,a.Xh,b)},null,null,3,0,379,23,[],"instance",358],
@@ skipping 7104 matching lines @@
 function Isolate(){var x=Object.prototype.hasOwnProperty
 for(var w in y)if(x.call(y,w))this[w]=y[w]
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 Isolate.prototype.constructor=Isolate
 Isolate.p=y
 Isolate.$finishClasses=a.$finishClasses
 Isolate.makeConstantList=a.makeConstantList
 return Isolate}}
 })()