Fix crashing bugs when dart code executed inside the debugger triggers breakpoints

LayoutTests/dart/inspector/evaluate-in-console-expected.txt

 CONSOLE MESSAGE: line 1: foo
 Debugger was enabled.
 
 Running: testScopeChain
 Script execution paused.
 Call stack:
     0) handleMessage (evaluate-in-console.dart:12)
 test.instanceField = "Test.instanceField"
 test.instanceField = "new value" = "new value"
 test.foo = null
 test.foo = 3 = 3
 test.foo = 3
+test.somePropertyThatDoesntExist = undefined
 test.toString() = "<Test instance with instanceField = new value>"
 test.toString(1) = "Unhandled exception:
 Closure call with mismatched arguments: function 'call'
 
 NoSuchMethodError: incorrect number of arguments passed to method named 'call'
 Receiver: Closure: () => String from Function 'toString':.
 Tried calling: call(1)
 Found: call()
 #0      Object.noSuchMethod (SOURCE_LOCATION)"
 test.create("x").instanceField = "x"
 test.concat(test.create("x"), test.create("y")) = "new value:x:y"
 _private = "private"
 globalField = null
 calculateSquareRoot(25) = 5
 new Element.tag("div").outerHtml = "<div></div>"
 new Element.tag("div").tagName = "DIV"
 a=new List() = 
 jsObject={} = Object {}
 jsObject.__proto__=a = 
 jsObject.add(7) = null
 a.toString() = "[7]"
 
 Running: testPrint
 Script execution resumed.
 Debugger was disabled.