I tested in Chrome and when style(line 50) is null, style.toString is undefined
Reason: typeof null equals "object" (at least in Chrome)

* fix colour settings of BeforeLabel and BeforeBody

* delete redundant variable declaration

* collect label colour setting.

Chrome specific issue that happens when destroying a chart and re-creating it immediately (same animation frame?). The CSS animation used to detect when the canvas become visible is not re-evaluated, breaking responsiveness. Accessing the `offsetParent` property will force a reflow and re-evaluate the CSS animation.

* Don't change minorFormat when determining label capacity

* Fix choosing of formatting unit

#4784)

* added scope for tooltip position mode call and added docs

* added test for positioner

* removed named func for lint

* resolved pull-request comments

Generate time scale ticks (`ticks.source: 'auto'`) based on the effective visualized range instead of the actual data range, meaning that the computed units and/or step size may change if the time options min and max are different from the data min and max.

These settings deal with browser disconnects. We had seen test flakiness from Firefox:
[Firefox 56.0.0 (Linux 0.0.0)]: Disconnected (1 times), because no message in 10000 ms

Fix Rollup issue caused by early access of the `extend` and `inherits` helpers not yet part of the `helpers/index` import. Also added (basic) unit tests for whose methods.